A worn camshaft is usually replaced. However, there may be instances when the camshaft will require straightening or rebuilding of the lobes. This may be due to having to customize the lobe lift and duration, or because the camshaft is not readily available.
Before attempting reconditioning of the camshaft, it is important to determine its construction material. camshaft are generally made of low carbon steel, high-carbon steel, chilled iron, or malleable or spheroidal iron.
- Low-carbon steel camshafts have too thin a case hardened to be ground without having to perform a hardening operation after they are reconditioned.
- High-carbon steel camshafts can be ground if none of the lobes have chips or excessive wear.
- If all of the lobes are worn, they should be ground until they all have the same profile.
- After profile grinding, the lobe surfaces are hardening by heating the area between 1,500° and 1,600° F, then cooling it in oil.
- Finish grinding of the lobe is performed after the lobe has been rehardened.
- Both types of cast-iron camshafts can be ground without having to reharden the surfaces.
- If these types of shafts require building up of the lobes (which is the preferred procedure when more than two lobes are worn), all oil must be removed before the welding process is started.
There are two common methods of reconditioning the camshaft lobes: grinding and building up.
Grinding removes the worn portions of the camshaft lobes, providing a smooth surface for lifter operation.
- The original camshaft lobe profile (lift and duration) can be achieved by removing the same amount of material from the nose circle and the base circle radiuses.
Grinding the camshaft lobe to restore original lift characteristics.
- If desired, you can customize the camshaft by changing lift and duration.
Grinding the camshaft lobe to change the lift and duration characteristics.
- Increased duration and lift are accomplished by removing more material from the base circle than the nose.
- The ramps can also be modified to provide for advanced or retarded valve opening.
When determining how much metal to remove from the nose and base, use the original specifications of the lobe. Since the lobe is worn, using actual measurements to determine material removal will result in undesirable lobe profiles.
If more than two lobes are worn, it is best to build up the lobes, then grind them.
- The result of this operation is camshaft lobes the same size and profile as the original.
- The lobes are first ground to undersize, then welded to build them up.
- Lobe configuration is restored by grinding the weld to the desire lift and duration characteristics.
- A master shaft is used to guide the grinding machine, much like a locksmith makes a key copy by following the form of the original key.
- To finish the procedure, the lobes are coated by immersion in a solution of molybdenum disulfide.
If undersize bearings are available, it is possible to recondition the camshaft journals to fit them.
- This would be done only if the journals are worn or scored.
- Most OHV (overhead valve) engines do not wear camshafts on the journals since there is very little load on them.
- The procedure for journal grinding is much like that of reconditioning main bearing journals of the crankshaft.
- Recondition all journals to the same undersize.
- Remember, many manufacturers use progressively smaller journal sizes on the camshaft.
- In this case, the journals are to be ground to the standard undersize, not to the same size.
If the camshaft is bent, it will have to be straightened before the lobes are reconditioned.
Camshaft straightening is done by relieving the stress in the metal, not by rebending it straight.
To peen the camshaft:
- Set it into a set of V-blocks and use a dial indicator to locate the point of greatest defection.
- With the high point of the camshaft facing down, use a bronze-tipped blunt chisel and hammer to relieve the stress.
Straightening a camshaft.
- Note you are striking in the direction of the bend, not against it.
- This shocking of the camshaft will result in the stress being removed and the camshaft returning to its original shape.
- Start peening at the journal indicating the greatest amount of TIR and work for this journal down the shaft in both directions to remove the warpage.
- Regularly, recheck your progress; the shaft will usually straighten quickly.
Do not attempt to straighten a camshaft by striking it against the direction of the bend. Doing this causes the shaft to be bent back into position and weakens the shaft.